This invention relates to a novel instrument and to a new and advantageous method afforded by that instrument for obtaining the X-ray diffraction data of a polycrystalline powder.
Conventional powder diffractometers or powder cameras register diffraction lines that represent a portion of the cone of diffracted rays emanating from a powder sample irradiated with a beam of monochromated X-rays. In the Debye-Scherrer-Hull technique, a pencil beam of monochromatic X-rays impinges upon a thin cylindrical specimen consisting of a very large number of tiny crystals of 0.001 cm or less diameter. The monochromatic radiation is usually the K.alpha..sub.1 .alpha..sub.2 doublet obtained by the use of an appropriate filter or a crystal monochromator. The geometrical factors determining the resolution of the lines in a Debye-Scherrer-Hull diffraction pattern are: R, the radius of the camera; r, the radius of the cylindrical sample; h, the length of the sample; .delta., the divergence of the impinging X-ray beam; and .sigma., the difference between the wavelengths of K.alpha..sub.1 and K.alpha..sub.2. For a fixed camera radius, the optimum resolution corresponds to the limit as r, h, .delta., and .sigma. all approach zero. However, under these conditions the exposure times become inordinately long.
For this reason, parafocusing cameras or diffractometers have gained preference. In the Bragg-Brentano configuration, a rectangular flat-faced briquet of powdered sample is irradiated by a divergent beam of monochromatic X-rays and a small portion of the total diffracted radiation is selected by an appropriately placed narrow receiving slit which lets the focused radiation enter a suitable X-ray detector such as a Geiger-Muller counter. During the exposure, the X-ray detector assembly is turned at twice the angular velocity of the turning sample holder in order to maintain parafocusing conditions. For precision measurements of interplanar spacings, the rectangular sample surface must be exactly tangent to the focusing circle at the center of the goniometer circle and must also be sufficiently short to be a good approximation to the parafocusing circle. Moreover, the sample should be very thin, approximately 0.05 mm thick for a sample having a linear absorption coefficient less than 50 cm.sup.-1. A highly objectionable feature of this flat surface specimen technique is the great tendency to preferred orientation of the crystallites and the resulting distorted relative intensities of the powder reflections.
Three principal disadvantages of conventional X-ray cameras or diffractometers are: (1) only a small fraction of the total diffraction cone for a particular set of interplanar spacings is measured; (2) the accuracy of the interplanar spacing measurements is markedly dependent on the Bragg angle of direction, i.e., .sup..DELTA.d /d=-cot.theta..DELTA..theta. where d is the interplanar spacing in Angstrom units and .theta. is the Bragg angle, and (3) the difficulty of achieving random orientation of the powder particles. Additionally, maximum line intensity and optimum resolution of lines in the diffraction pattern cannot both be achieved by known diffractometers. A major object of this invention is to provide a parafocusing powder diffractometer that avoids these disadvantages. Other objects and advantages of the present invention will be apparent from its description which follows.